This invention relates to a system and method for controlling a fuel cell-powered system. More particularly, the invention relates to a predictive control arrangement for load-following fuel cell-powered applications and systems, such as vehicles and the like, which require rapid but not instantaneous response to operator requests.
Fuel cell power systems typically include one or more cell stack assemblies (CSA) supplied with a fuel reactant and an oxidant reactant, typically air, to provide an electrochemical reaction process to generate electricity. A coolant, such as water, may also be present as part of the process in the CSA, and water/steam may also be used the fuel supply process. Regulation of the reactant supplies and the ancillary coolant and/or water/steam supplies is typically used to control the process within the CSA and the operation of the fuel cell power plant.
Fuel cell power plant controls for stationary applications may be load-following or non-load following, usually depending upon whether the system is grid-connected or grid-independent. In the grid-independent mode, power plants must follow load changes instantaneously to prevent unacceptable voltage excursions. On the other hand, in other applications such as for powering the traction motor of a vehicle, the controls are required to be load following but need not be instantaneous to provide acceptable response from an operator standpoint. indeed, some degree of control resides in the operator who chooses the broad operating parameters of the system, such as changes in vehicle speed, temperature control, and the like. To the extent very gradual changes in the load are to be commanded, a relatively basic and simple control system may be suitable. On the other hand, because fuel cell response is instantaneous to increased demands in current output or load, the fuel cell may encounter a condition of reactant starvation, and in some instances even insufficient cooling, if the current or power demand exceeds the rate at which changes are, or can be, made in the supply of reactants or coolant to the CSA.
One such example of the challenge that may be encountered during a condition of transient load increases is described in U.S. Pat. No. 4,729,930 to Beal et al. There, a constant speed blower normally provides an adequate reactant air supply via a single modulated valve except during significant transient load increases. Then, the control system of that patent provides for energizing one or more quick opening auxiliary air valves in parallel with the modulated valve to promptly deliver an increased supply of air from the blower to the CSA during such transients. This provides an adequate supply of reactant air during load transients, but at the xe2x80x9ccostxe2x80x9d of a large fixed-speed blower and an array of controlled, rapidly opening valves
Yet other patents, such as U.S. Pat. Nos. 5,771,476 and 5,991,670 to Mufford et al, describe systems for controlling the electric power output and oxidant supply in a fuel cell system for vehicles, particularly as a result of anticipated load demand. Those patents concern a system in which a variable speed compressor supplies the oxidant and comprises a very significant load in addition to the vehicle""s traction motor. The electric power output of the CSA is dependent on the compressor""s speed. To eliminate compressor revving and instabilities common to prior systems, a control system is provided which comprises a summing device for determining the total instantaneous power demand of the electrical loads based on a plurality of sensed power demand signals, and a processor for generating a feed-forward output signal for adjusting the compressor speed to a value predicted to give fuel cell power output sufficient to satisfy the instantaneous power demand. Although addressing the problem of compressor revving and instability, this approach continues to run the risk of starving the CSA of one or more of its process-controlling components, such as oxidant reactant, if the load transient increases, or is allowed to increase, at a rate that is too rapid relative to the process-controlling variable(s) being controlled.
In view of the foregoing, it is an object of the invention to provide a predictive control arrangement for following load transients in a load-following fuel cell system which minimizes or eliminates starvation of the CSA during load transients. Other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.
The present invention is an arrangement for controlling one or more fuel cell stack assemblies (CSA) in a fuel cell power plant in load-following applications, such as powering automotive vehicles, in a manner which reduces, minimizes, or eliminates CSA starvation during electrical load transients.
Accordingly, the present invention comprises a method and system for controlling a fuel cell power plant in a predictive manner providing rapid response of the CSA-powered system without creating an unacceptable condition of CSA starvation which may be caused by instantaneous electrical load transients. The control system and method of the invention are for use in a fuel cell-powered system having a CSA for providing electrical power to one or more electrical loads controllably connected thereto and having respective sources of fuel and oxidant reactants and coolant for effecting the operation, or process, of the CSA. The method of controlling the system in an anticipatory manner to minimize or eliminate reactant starvation and/or insufficient cooling during load transients comprises providing a demand signal representative of the anticipated current or power required by the one or more electrical loads; providing a load signal representative of the actual current or power drawn by the one or more loads connected to the CSA; selecting the greater of the demand signal and the load signal and providing a control signal for regulating one or more of the reactants and coolant to effect the process of the CSA; providing one or more process status signals indicative of the status of the process effected; converting each of the one or more process status signals to a respective load capability signal; and selecting the lesser of the demand signal and each of the respective load capability signals to provide an output signal for commensurately controlling a system load.
Further, the step of providing a control signal comprises transforming the selected load signal to a nominal setpoint signal as a function of the correlation between load current and the respective one or more of the reactants and coolants to be regulated. That step of providing the control signal may further comprise comparing the selected and transformed nominal setpoint signal with one or more process feedback signals, such as status signals of the operating process effected, to thereby provide an error signal, and summing a function of the error signal with a function of the selected and transformed nominal setpoint signal.
A corresponding control system, including a suitable controller, is provided to effect the steps of the method of controlling the CSA and associated loads as mentioned above.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.